Abstract The T-box family of transcription factors is a group of proteins that regulate a host of developmental processes, such as tissue specification and migration, organ patterning and morphogenesis, and limb identity and outgrowth. Loss of T-box factor function is linked to a wide variety of developmental disorders, including congenital heart defects, craniofacial deformities, and limb malformations, as well as several types of cancer. Eomesodermin (Eomes), a member of the T-box family, is itself associated with a variety of disease states, such as microcephaly, lymphoma, and colorectal cancer. During embryonic development, Eomes is required in a TGF- signaling-associated manner for the earliest stages of vertebrate mesoderm and endoderm specification. Recently, Eomes was shown to directly activate Mesp1, a gene believed to be the master regulator of the cardiogenic transcriptional program, and may do so independently of TGF- signaling. In contrast, Eomes is also required in different tissue contexts for proper forebrain and immune system development. Therefore, Eomes' functional specificity relies not only on the tissue-specific targets it regulates in a given cell, but on the transcriptional co-regulators with which it interacts. I am using an immunoprecipitation-based proteomic approach coupled with biochemical assessment to study the compositions of Eomes- containing transcriptional complexes during early tissue specification. I am also developing in vivo Eomes loss-of-function models in two species of frog, Xenopus laevis and Xenopus tropicalis, to investigate the endogenous functions of Eomes during embryonic development. This work will serve to identify novel coregulators that contribute to Eomes' tissue-specific functions in the developing endoderm and cardiac mesoderm. It will also serve as a foundation for use in identifying novel mechanistic and therapeutic targets for diseases previously associated with loss of Eomes functionality, including immune deficiencies and lymphomas, and potentially congenital heart defects.